Contingency management (CM) interventions are highly efficacious in improving substance abuse treatment outcomes. These interventions have been provided primarily in an individual format, but most therapy for substance abusers is delivered in the context of groups. We have preliminary data suggesting that our prize-based CM, which is substantially less costly than traditional voucher-based CM, can be administered in a group context. In this application, we propose to evaluate the efficacy of prize-based CM when administered exclusively in a group setting. Substance dependent patients beginning intensive outpatient day treatment (N=360) at one of three community-based programs will be randomly assigned to one of two conditions: (a) standard, non-CM treatment or (b) standard treatment plus prize CM delivered in groups. In the CM condition, patients will earn the opportunity to win prizes ranging from $1 to $100 in value for attending groups and submitting drug-free biological specimens. Substance use and psychosocial problems will be measured at intake, at month 1, at month 3 (post treatment), and at 6-, 9-, and 12-month follow-up evaluations. We will also assess patient characteristics that may be associated with improved outcomes within and across conditions. We will evaluate the cost-effectiveness of group-based CM by assessing receipt of psychosocial and medical services and criminal justice system involvement throughout the treatment and follow-up periods.